Optical telecommunications over optical fibers is now the preferred mode of high-bandwidth data transmission in comparison to copper wire, particularly over long distances. Such systems use lasers modulated in amplitude by the data to be transmitted. The signals are coupled into an optical fiber for detection and demodulation at the other end of the link. The existing infrastructure of long-haul optical fiber is rapidly becoming taxed to its bandwidth capacity. Laying more fiber to carry additional bandwidth is an extremely expensive proposition.
Dense wavelength-division multiplexing (DWDM) has emerged as a more cost-effective solution. The idea is to force existing fibers to carry more bandwidth by combining signals from multiple lasers operating at different wavelengths onto a single fiber. Key components of DWDM systems include the optical multiplexers and demultiplexers, the latter often being the former operated in reverse. The multiplexers take optical signals at different wavelengths propagating on different fibers and combine them onto a single fiber. The demultiplexers take several wavelengths propagating on a common fiber and separate them onto different fibers.
Another important component in a DWDM system is the add/drop module, or OADM. The OADM is used to drop or pick-off wavelengths 102 to carry local node traffic to businesses or other destinations for optical or electro-optical conversion. The OADM is also used to re-insert wavelengths 104, typically carrying new data, back into the DWDM fiber(s). These functions are illustrated schematically in FIG. 1. An incoming fiber 110 carries multiple DWDM wavelengths 112 into the module, and an outgoing fiber 120 outputs the modified DWDM traffic 122. Unmodified or express wavelengths are depicted at 130.
As shown in FIGS. 2A and 2B, existing OADMs utilize passive components to define a fixed wavelength or a set of wavelengths to be dropped or added. FIG. 2A shows a single-wavelength schematic, having a fixed fiber Bragg grating 202 to drop and add λi, whereas FIG. 2B illustrates a multiple wavelength schematic to add and drop two fixed wavelengths, λi and λj using Bragg gratings 212 and 214. Items such as 202 represent circulators. FIG. 3 depicts a wavelength-configurable system including input wavelength multiplexers 302 and output demultiplexers 304 on the other side of a fiber-optic crossbar switch 310. A network control function 312 is used to dictate which channels are dropped and added according to the switch settings.
Although configurations of the type just described have been in use for some time, they are inflexible and/or expensive due to the discrete nature of the components involved. As such, systems based on these concepts tend to be expensive to implement and maintain. Accordingly, there remains a need for a more flexible, easier to implement OADM for use in DWDM and other applications.